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ELECTRICAL DRIVES

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Magnets are pieces of metal that have the ability to attract other metals. Every magnet has two poles: a north and a south. Much like electrical charges, two similar magnetic poles repel each other; while opposite magnetic poles attract each other. Magnets have a continuous force around them that is known as a magnetic field. This field enables them to attract other metals. Figure 1 illustrates this force using bar and horseshoe magnets. | A phase-controlled cycloconverter can be used for speed control of an ac machine (induction or synchronous type). Figure 8 shows a drive using a three-pulse half-wave or 18-thyristor cycloconverter. Each output phase group consists of positive and negative converter components which permit bidirectional current flow. The firing angle of each converter is sinusoidally modulated to generate the variable-frequency, variable-voltage output required for ac machine drive. Speed reversal and regenerative mode operation are easy. The cycloconverter can be operated in blocking or circulating current mode. In blocking mode, the positive or negative converter is enabled, depending on the polarity of the load current. In circulating current mode, the converter components are always enabled to permit circulating current through them. The circulating current reactor between the positive and negative converter prevents short circuits due to ripple voltage. The circulating current mode gives simple control and a higher range of output frequency with lower harmonic distortion.